Z. Ren , Wan, W., Xiong, J., and Liu, L.
Abstract:
In this work, we investigated the equinoctial asymmetry of the ionospheric vertical ExB plasma drift velocity (V) in the equatorial F region. Based on observations from ROCSAT-1 during 1999 to 2004, we found that the observed asymmetry exhibits obvious local time dependence with three noticeable features. First, in the Eastern Hemisphere during the interval between 0900 and 1300 LT, V is obviously higher at the March equinox (March-April) than at the September equinox (September-October). Second, there is a pronounced asymmetry for wave number 4 longitudinal structures of V. Third, around sunset we find an obvious asymmetry of the prereversal enhancement (PRE).On the basis of the drift observation mentioned and TIME-IGGCAS model, we simulate the effect of the V asymmetry on the ionospheric plasma density. It is found that the daytime V asymmetry can partly explain the equinoctial asymmetry in daytime low-latitude ionospheric plasma density observed by Liu et al.Using TIDM-IGGCAS-II model and tidal winds from TIMED/TIDI observations, we also simulated the influence of the lower thermospheric tidal winds on the equinoctial asymmetry of V. The simulated equinoctial asymmetry in V are mainly driven by DW1, SW2, DE3, and DW2 tides. The asymmetry in daytime V varies with local time and longitude and mainly shows three features. First, the simulated daytime V during the March equinox is larger than that during the September equinox in most of longitudinal sectors. Second, the daytime V asymmetry in the Eastern Hemisphere is more significant than that in the Western Hemisphere. Our simulation suggests that the longitudinal variations of the geomagnetic fields and DW2 tides play important roles in the generation of this hemisphere difference. Third, there is an obvious wave number 4 longitudinal structure in the V asymmetry driven by the equinoctial asymmetry of the DE3 tide.
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